After graduating from law school, Peterson worked as a lawyer in private practice in Eau Claire, Wisconsin, and was active in the state legal community.
In addition to his legal career, Peterson worked part time as a member of the faculty of the University of Wisconsin-Eau Claire from 1976 to 1978 and then served as a court commissioner from 1978 until his appointment to the judiciary.
One of his most noteworthy cases after retirement was his involvement in the John Doe investigation of Governor Scott Walker for illegal campaign coordination with conservative political action committees.
In January 2014, Judge Peterson made the consequential decision to void several subpoenas granted by Kluka, ruling that it was not clear that the coordination alleged by the prosecutor constituted an actual crime.
[7] Although Peterson stayed his ruling pending appeal, the decision effectively stalled the investigation until after the 2014 gubernatorial election, where Walker narrowly won re-election.
He led the state's delegation to the 1968 Democratic National Convention in Chicago, where he supported anti-war candidate Eugene McCarthy and joined protests taking place outside the venue.